Nitrosomonas europaea strain Schmidt produces at least three acyl homoserine lactone (AHL) signal molecules: C 6 -homoserine lactone (HSL), C 8 -HSL, and C 10 -HSL. These compounds were identified in extracts of chemostat culture effluent by three independent methods. The concentrations of AHL in effluent were low (0.4 to 2.2 nM) but within the range known to induce AHL-responsive systems. The absence of LuxI and LuxM homologs from the genome of N. europaea strain Schmidt suggested that AHL synthesis occurs by an alternate pathway, possibly mediated by an HdtS homolog. To the best of our knowledge, the present report is the first to document the types and levels of AHLs produced by N. europaea.Nitrosomonas europaea and other ammonia-oxidizing bacteria (AOB) play a pivotal role in affecting the fate and behavior of nitrogen in the environment. Terrestrial and freshwater AOB grow predominantly in biofilms (1,7,13,18), but little is known about how conditions unique to the biofilm environment affect their biology and, consequently, nitrogen cycling. In other members of the class Proteobacteria, a major process affecting cellular structure and function in biofilms is quorum sensing, which is mediated by acyl-homoserine lactone (AHL) signal molecules (6). Much is known about AHL production by a diversity of heterotrophic proteobacteria, but to date, AHL production by the chemolithotrophic AOB has not been conclusively documented. The objectives of this study were to determine if N. europaea strain Schmidt produces AHL and, if so, to identify the types and levels of these molecules.N. europaea strain Schmidt was obtained from the American Type Culture Collection (ATCC strain 19718). Batch cultures were grown in ATCC medium 2265 (2) at 25°C in lightshielded flasks to which aliquots of sterile 30% (wt/vol) K 2 CO 3 were periodically added. Continuous culture of N. europaea was done in a BioFlo 110 modular benchtop fermentor (New Brunswick Scientific Co., Edison, NJ). Cells were grown in ATCC medium 2265 (lacking cresol red) in a light-shielded vessel at a specific growth rate of 0.025/h (71% of the maximum specific growth rate reported for N. europaea [9]). The temperature was held at 25°C, the dissolved oxygen concentration was maintained at 5 mg/liter (60% of the saturation level at 25°C), and the pH was maintained at 7.1 Ϯ 0.1. A nearneutral pH was used to minimize the potential for AHL degradation by lactonolysis (20). At steady state, the culture optical density at 600 nm was 0.11 (6 ϫ 10 6 cells/ml, 45 g biomass dry weight/ml). Aliquots (100 l) of the culture were regularly plated on 1/10 nutrient broth to verify the absence of heterotrophic contaminants.Extracts were prepared from supernatants of N. europaea batch cultures (5 liters total) and chemostat effluent. Batch cultures were harvested when the cultures had twice acidified their medium, after which accumulation of nitrite-limited growth (final optical density at 600 nm ϭ 0.08). For the chemostat culture, 7 liters of effluent was collected, the cells were r...

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Cropping systems may improve or decrease soil quality depending on the specifi c crop rotation, nutrient amendments, and tillage practices employed. We conducted this study to determine the eff ect of six cropping systems in the Wisconsin Integrated Cropping Systems Trial on soil properties aft er 18 yr of continuous treatments. We sampled soils (0-5 cm and 5-20 cm depths) following the corn (Zea mays L.) year of three grain-based systems (continuous corn and two grain rotations), aft er both corn and alfalfa (Medicago sativa L.) in two forage-based systems (organic and conventional), and in grass-legume pasture. Extractable P and K, pH, total organic carbon (TOC), total N, active soil C, potentially mineralizable nitrogen (PMN), water-stable aggregates (WSA), bulk density (BD), penetrometer resistance, and total microbial biomass (TMB) were measured, and the Soil Management Assessment Framework (SMAF) soil quality index (SQI) was determined. Th e pasture (0-5 cm) was signifi cantly better than all other systems in almost all soil quality indicators and had the highest SQI (96 vs. mean of 87 for others). Th e alfalfa-based systems had more total N, TOC, active C, PMN, and WSA and higher BD in one or both depths than did the grain-based systems but SQIs were not diff erent. Among the grain systems, there was less variation and few signifi cant diff erences were observed. While there were signifi cant diff erences among systems for most soil properties, a SQI based on a composite of seven soil properties, showed few diff erences on this well managed, productive, prairie-derived soil.* Signifi cance of contrast: P = 0.05. ** Signifi cance of contrast: P = 0.01. + Signifi cance of contrasts: P = 0.10. † Means are LSMeans.

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New spectrophotometric bioassay procedures have been developed for evaluating chemical toxicity, using electron transport particles isolated from bovine heart mitochondria, based on the ability of many toxic chemicals to interfere with the integrated function of electron transport enzymes. The sensitivity of the mitochondrial assay is compared to published sensitivities of other in vivo and in vitro toxicity testing methods. Regression analysis of logarithmically transformed toxicity values for 42 chemicals, including 8 pesticides, 5 drugs, 6 metals, 8 alcohols, 5 respiratory inhibitors, 4 phenols, and 2 phthalates, indicates excellent correlation between the sensitivity of the new assays and the sensitivity of mammalian cytotoxicity studies (r2 = 0.86). Data from aquatic exposure toxicity tests conducted in fish are also highly correlated with the mitochondrial assay results (r2 = 0.79). However, correlation of data from these methods with median lethal dose studies conducted in rats is not as good because of the inability of in vitro and aquatic exposure analyses to account for the gastrointestinal absorption, hepatic metabolism, and excretion processes which modify toxic responses following oral administration.